Device and method to eliminate step response power supply perturbation

ABSTRACT

A system and method for eliminating step response power supply perturbation during voltage island power-up/power-down on an integrated circuit is disclosed. An IC chip communicates with a primary power supply and includes at least one voltage island. A primary header on the voltage island of the chip communicates with the primary power supply via a primary header power path. A secondary header on the voltage island of the chip communicates with a secondary power supply via a secondary header power path. A control decoder communicating with the IC chip and the voltage island regulates the state of the primary and secondary headers.

FIELD OF THE INVENTION

The invention relates to a voltage island device on an integratedcircuit, and more particularly, to a voltage island device foreliminating step response power supply perturbation during voltageisland power-up/power down.

BACKGROUND OF THE INVENTION

Three principle sources of power supply perturbation exist withintoday's integrated circuits: resistive voltage (IR) drop; cyclic powersupply ripple; and DI/DT induced supply bus ringing. IR drop is causedby the DC current demand of the Integrated Circuit (IC) and resistanceof the power supply network within the IC and results in both reducedvoltage at the operating circuits relative to the voltage supplied at IClevel and voltage gradients across the IC. Cyclic power supply ripple isgenerated by clocked logic operating on the IC which creates periodic ACcurrent variation and resultant voltage ripple. DI/DT induced supply busringing is caused by near-instantaneous current demand changes during ICpower-up or when voltage island portions of the IC are enabled/disabled.These step response current changes excite theresistance/inductance/capacitance (RLC) network of the power supply busgenerating a voltage ring that is damped over a transient time periodwhich is much greater than the operational period (1/OperationalFrequency) of the IC.

Current techniques for reducing DC/Ripple perturbation include quietcapacitance modeling and estimation, automatic power supply busrouting/generation and DC current modeling/load rebalancing. However,step response remains a serious problem in today's IC chips becausevoltage extremes produced during the step response dictate the minimumand maximum operating voltages encountered within an IC. In prior ICdesigns, the IC was powered up and down in total, and a wait time andreset performed after power bus stabilization to mitigate power-ontransient response. Current IC designs increasingly provide voltageislands which are powered up and powered down multiple times while theremainder of the IC remains functional. A voltage island (VI) allows forone or more portions (islands) of an integrated circuit chip to beselectively powered by either a chip-wide power source or other voltageisland power source. Voltage island power sources can be switched on andoff in accordance with the operational demands of the integrated circuitrequiring supply current from the chip-wide power source and consumingpower only when required for functional operation of the IC.

However switching of the VI power supply places the IC at risk offailure due to step response voltage transients and their inherentoscillation frequencies. Each time an island is powered up, the initialcurrent requirements needed to charge diffusions and wells within theisland must come solely from on-chip quiet capacitors associated withthe chip-wide power source, as the inductive properties of the packageprevent real-time charge replacement. These sudden changes in currentrequirements generate a step response in the chip-package power busnetwork which results in damped ringing of the power bus voltage, i.e.,power supply perturbation.

Referring to FIG. 6, circuit diagrams 500 of the T0− and T0+ powersupply response due to voltage island power-on is shown. Just beforepower on, the bus system is stable and the quiet capacitance 504 in thecircuit 502 is fully charged. The rate of change in current flow throughthe inductor 506 to Cq is 0. At time 0, voltage island capacitance, Cs510, is switched onto the bus using switch 508 and begins to charge tothe rail value. Charging of Cs 510 presents an instantaneous currentdemand in the circuit 502, or high di/dt to the V_(DD) bus, however, dueto the inductive component 506 of the bus, the initial di/dt currentremains 0 and all charge pulled into Cs (510) is pulled out of Cq (504)in a charge sharing event. The result is that the voltage across Cq(Vcq) falls. The ratio of the Vcq at T0+ relative to Vcq at T0−illustrated in FIG. 6, is represented by the equationV_(DD)′/V_(DD)=Cq/(Cq+Cs) and is directly related to the capacitance ofthe voltage island relative to the IC remainder. Thus, an undesirablestep response results which produces unwanted power supply perturbation.

The oscillation produced by the di/dt event excitement of the ICchip/package RLC network is typically in the 80 MHz to 200 MHz range inpresent day semiconductors. These frequencies are centered around clockfrequencies typically encountered in ICs for both data processing andI/O functions which further compromises functionality of the IC as thevoltage oscillation affects path delays within the IC from cycle tocycle. Past IC chip designs have been unable to adequately resolve theundesirable step response issue. With advances in IC technology and thedrive to higher performance and lower power products, integratedcircuits are being designed to lower rail voltages to power the chip.These challenges require even more attention to power distribution inthe chip and the affects of step response perturbation at each newtechnology node.

Referring to FIG. 4, a typical known semiconductor IC module 300including two voltage islands 310, 320 is shown. A power supply V_(DD)is brought from off-module/chip from a plurality of V_(DD) inputs 302which are shorted together at chip level to form a robust powernetwork/bus 308. Logic circuits, are distributed throughout the IC/chip306 and attached to the power network. The two voltage islands 310, 320are also powered by the power network 308, however, power to theseislands of logic function 310, 320 is gated using header circuits 314,322, respectively, disposed between the global power supply network ofthe IC chip/module 308 and the local power supply network of the voltageislands 310, 320. Each voltage island implemented within the IC/chip 306may be controlled independently with regard to its power up/downcondition at any time during IC function.

Referring to FIG. 5, the electrical equivalent of a typical IC RLCnetwork 400 is shown. It is understood the logic functions connected tothe power supply network consume some amount of functional current, andare modeled as capacitor components Cq 410 that comprise diffusions andwells within the circuits which provide storage capacity for chargeassociated with the power supply voltage V_(DD) 404. As a result,circuits are modeled as the Cq component 410 of the power supplynetwork. The summation of module, image and circuit contributions yieldsan RLC network 400 for the power bus. Similar to the logic outsidevoltage islands, logic inside a voltage island is modeled as acapacitance 422, however, as these supply capacitances are charged anddischarged in response to a power supply switch (header) they aredefined as switching capacitance (Cs) 422. The typical IC RLC network,as shown in FIG. 5, has disadvantages. For example, when power isbrought in from off module/chip 402, and the voltage island switch 420is engaged, unwanted inductance associated with the IC package and ICimage is encountered. Additionally, while the power supply system withinthe IC is quite robust, there remains some amount of resistanceassociated with each segment of the bus 406 that affects both thetransient and DC response of the power supply network.

Presently, noise reduction during voltage island turn on is managedthrough design of the header circuits which gate power to the voltageisland. Headers are designed to provide a time-decreasing impedancebetween the power bus and island to slow charge up, and therefore reduceunwanted current step response (di/dt).

A block diagram for a prior art header circuit 550 is shown in FIG. 7.The header circuit 550 includes voltage supply V_(DD) 560 and headercontrol input 552 which provides selective coupling of voltage supplyV_(DD) 560 to Voltage Island Power Supply Rail 561 through a pluralityof power supply coupling switches 554. Built-in delay units 556 timeseparate activation/deactivation switches 554 in response to controlinput 552 to limit di/dt during the voltage island power-up/power-down.In addition to header control input 552, typical voltage islands alsoinclude at least a second control signal to control the logic boundaryof the VI independent of the header control 552. Design of thesestructures is complex and may not always optimize turn-on time for theinternal capacitance of the voltage island. Thus, typical headercircuits have not been successful in mitigating RLC-induced power supplyringing caused by current step response during voltage islandpower-up/power-down. Thus, there is a need for a circuit structure andmethod for coupling a voltage island to a semiconductor power bus whichmore effectively eliminates the step response and unwanted power supplyperturbation of conventional IC designs.

SUMMARY OF THE INVENTION

The invention relates to a circuit for reducing noise in an integratedcircuit having one or more voltage islands. Provided is a first powersupply directly coupled to logic circuits exclusive of voltage islandswithin the IC and a primary circuit comprising a first switch forselectively coupling first power supply to voltage island logiccircuits. A pre-charge power supply provides an alternate power supplybus to the voltage island logic circuit and a secondary circuitcomprised of a second switch selectively couples the pre-charge powersupply to the voltage island logic circuit. The pre-charge power supplypowers the voltage island logic circuit from an off state into apre-charged state and the first power supply powers the voltage islandlogic circuit during operation.

In a related aspect of the invention, the pre-charge power supplyconnects to a plurality of secondary circuits.

In a related aspect of the invention, the pre-charge power supplyconnects to a plurality of secondary circuits on a plurality ofintegrated circuits.

In a further aspect of the invention, a device is provided foreliminating step response power supply perturbation during voltageisland power-up/power-down on an integrated circuit. An IC chipincluding a primary power supply and a secondary power supply where theIC chip includes at least one voltage island. A primary header on thevoltage island of the IC chip communicates with the primary powersupply. A secondary header on the voltage island of the IC chipcommunicates with the secondary power supply. A control decodercommunicates with the IC chip and the voltage island for regulating thestate of the primary and secondary headers.

In a related aspect of the present invention, the primary header on thevoltage island of the IC chip communicates with the primary power supplyvia a primary header power path on the IC chip. Also, the secondaryheader on the voltage island of the IC chip communicates with thesecondary power supply via a secondary header power path on the IC chip.

In a related aspect of the present invention, the control decodersequentially enables the secondary header on the voltage island whilethe primary header on the voltage island is disabled. The controldecoder disables the secondary header on the voltage island and nearsimultaneously enables the primary header on the voltage island.

In a related aspect of the present invention, the secondary power supplyconnects to a plurality of secondary headers.

In a related aspect of the present invention, the secondary header powersupply connects to a plurality of secondary headers on a plurality of ICchips.

In a related aspect of the present invention the device includes aheader control system for managing at least one voltage island includingthe primary and secondary headers.

In a related aspect of the present invention, the header control systemprioritizes initial power requests from multiple voltage islands.

In a related aspect of the present invention, the device includes aheader control system for managing multiple voltage islands whichinclude the primary and secondary headers. The voltage islands share thesecondary power supply via the secondary header power path such that theheader control system limits loading of the secondary power supply.

In a related aspect of the present invention the voltages on the primaryheader and the secondary header are substantially the same.

In a related aspect of the present invention the control decoderprevents disabling the secondary header on the voltage island and nearsimultaneously enabling the primary header on the voltage island duringa period when noise is on the secondary header or the secondary voltageis not substantially equal to the primary voltage.

In a further aspect of the present invention, a method is provided foreliminating step response power supply perturbation during voltageisland power-up/power-down on an integrated circuit. The methodcomprises the steps of providing an IC chip communicating with a primarypower supply, and the IC chip including at least one voltage island.Providing a primary header on the voltage island of the chip, where theprimary header communicates with the primary power supply via a primaryheader power path. Providing a secondary header on the voltage island ofthe chip where the secondary header communicates with a secondary powersupply via a secondary header power path. Providing a control decodercommunicating with the IC chip and the at least one voltage island. Themethod further comprises the steps of requesting an initial power-up ofthe voltage island, and enabling the secondary header using the controldecoder. The secondary header is then disabled using the controldecoder, and the primary header is enabled using the control decoder.

In a related aspect of the present invention, the secondary header isdisabled and the primary header is enabled near simultaneously.

In a related aspect of the present invention, the method furthercomprises a power down sequence including a series of stepped reductionsin current demand from the voltage island when the primary header isswitched from enabled to disabled using the control decoder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an Integrated Circuit (IC) according to thepresent invention depicting a header and a pre-header on a voltageisland;

FIG. 2 is an electrical equivalent of the IC shown in FIG. 1;

FIG. 3 is a block diagram of a voltage island header according to thepresent invention;

FIGS. 3A-3C are schematics of the circuit according to the presentinvention;

FIG. 4 is a block diagram of a prior art IC;

FIG. 5 is an electrical equivalent of the prior art IC shown in FIG. 4;

FIG. 6 is a schematic of a circuit according to the prior art;

FIG. 7 is a block diagram of a prior art header;

FIG. 8 is a block diagram of an embodiment of a control decoder;

FIG. 9 is a table and state diagram of an embodiment of control flowaccording to the present invention;

FIG. 10 is a block diagram of an embodiment of a control system; and

FIG. 11 is a circuit diagram depicting a power down embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a new header design and image/package busplan which eliminates the voltage island power-up issue of a stepresponse during power up (di/dt) resulting in power supply perturbation.In addition, the present invention results in both efficient managementof voltage island power state transition and efficient centralmanagement of voltage island power up. Further, the present inventionteaches a topology for mitigation of island power-off (di/dt) transientmitigation.

The present invention provides a power on methodology for an IC voltageisland having primary and secondary header circuits where the secondaryheader circuit is enabled and the island is pre-charged/repowered in astatic mode. Then, the secondary header is disabled and the primaryheader is enabled to provide connectivity to a power bus with functionalcurrent-load capability. Finally, the island data/clock boundaries areenabled to start IC functional processing.

Referring to FIG. 1, an integrated circuit/module 10 designed inaccordance with the present invention is shown. Integratedcircuit/module 10 contains integrated circuit chip 14 connected to themodule 10 using, for example, bond pads 12. In the present invention,each voltage island 40 within the chip 14 is provided with a headercircuit or primary header 42 for selective connection to the primary orfunctional IC power supply network 30 via bus 18. Additionally, thevoltage islands 40 include a pre-charge, or secondary header 44 forconnection to a second, less substantial, secondary or pre-charge powersupply network 34 which may connect to all voltage island secondaryheaders in the IC via a secondary or pre-charge header power path 36.The pre-charge power supply 34 is provided with its own off-module path(not shown) to provide isolation from the primary network.

Referring to FIG. 2, the electrical equivalent of FIG. 1 in accordancewith the present invention is shown. The RLC network for integratedcircuit/module 100 of FIG. 2 includes a secondary header switch 128according to the invention to connect each voltage island 120 switchingcapacitance to the pre-charge power supply network/pre-charge VDD 106.As power is brought in from off module, inductance associated with theIC module package 100 and IC 102 power supply connections image isencountered. Additionally, while the power supply system within the IC102 is quite robust, there remains some amount of resistance associatedwith each segment of the bus 108 that affects both the transient and DCresponse of the power supply network. The logic functions connected tothe power supply network/bus 108 consume some amount of functionalcurrent, thus, they are modeled as capacitances/Cq component 112 due todiffusions and wells within the circuits which provide storage capacityfor charge associated with the power supply V_(DD) 104. The summation ofmodule, image and circuit contributions yields an RLC network 100 forthe power bus 108. Circuit logic inside a voltage island 120 is modeledas a capacitance in a manner common with logic functions 112, however,as these supply capacitances are charged and discharged in response to apower supply switches 124 and 128 of FIG. 2 which represent primaryheader 42 and secondary header 44 of FIG. 1, they are defined asswitching capacitance (Cs).

The circuit logic outside and inside the voltage islands is fabricatedusing transistors and could have been depicted as such in FIGS. 2 and3A-3C. However, charging and discharging of the voltage islands is inview in the present invention, and as such the circuit logic is depictedas capacitance for illustrative purposes as discussed above. Thus, thecircuit logic within the ASIC, the header circuits, and the voltageislands is depicted in block diagram form in FIG. 1 and the circuitlogic is illustrated using capacitance in FIG. 2 and FIGS. 3A-3C.

The secondary or pre-charge power supply network shown in FIG. 2 iscomprised of inductive and resistive components representing module andchip components, however, it is not required that the primary andsecondary bus networks be electrically equivalent.

In the present invention, the primary or functional power supply network104 is sized to provide required load currents for functional operationof the IC 102 at low voltage droop (IR drop due to supply networkresistance). The primary header circuit is for connecting the voltageisland 120 to a primary power bus 108 in a functional mode, and thus,the header circuit and primary bus are capable of conducting a largeamount of current with low IR drop. In addition, it is also advantageousfor the IC interface between IC module 100 and IC chip 102 interface forthe primary supply 104 to be made using multiple package-levelconnections which lowers the effective inductance of the primary supplynetwork.

In contrast, the secondary supply network 106 does not have similarrequirements. The secondary header 128 is intended to provide initialcharging of the voltage island 120 Cs capacitance, therefore, current onthe secondary supply 106 is limited to initial charging current(transient) and leakage (DC) current associated with the island. Thesecondary header and bus are sized for pre-charging the voltage islandand to handle the leakage current of the island with a low IR drop. Thesecondary bus provides a separate path to chip/module level for thepower supply.

As a result, the resistive component of the secondary power supply 106including both the supply network and header on-resistance is onlylimited by the acceptable DC IR drop and associated voltage deltabetween the primary and secondary supplies 104, 106, caused by voltageisland 120 DC leakage. Additionally, as the bus is intended to operateduring power-up of the voltage island prior to functional use of theisland, the inductance of the secondary network may be ignored withsufficient allowance for settling time between pre-charge initiation andfunctional switch-over. As a result, the secondary supply may beimplemented with little additional power overhead.

Referring to FIG. 3, the voltage island header 200 of the presentinvention includes both a primary 204 and secondary 220 header circuit.The primary header circuit 204 is sized to reduce the on-resistance ofthe connection, limiting functional mode IR drop between the primarysupply network 208 and the voltage island supply network 250. Theprimary header circuit 204 of the present invention does not requirestaged turn-on to limit primary supply network 208 di/dt and associatednoise which greatly simplifies the design of the circuit.

Referring to FIG. 3, control signals 212, 230 of the primary 204 andsecondary 220 header circuits, respectively, are generatedindependently, but are not necessarily external to the header circuits.The secondary header circuit 220 may be much smaller than the primaryheader 204. The power supply network and header architecture of thepresent invention allows for the independent optimization of multipledesign requirements.

Although FIGS. 1, 2 and 3 detail topologies associated with a powersupply network, one skilled in the art would recognize that these sametopologies are applicable to ground networks and that provisions forprimary or secondary resources on the power supply, ground or bothnetworks may be advantageous in separating power-on noise from thefunctional bus envelope.

Referring to FIGS. 3A-3C equivalent circuit diagrams according to thepresent invention are shown depicting three states of the primary andsecondary headers of the voltage island according to the presentinvention. Referring to FIG. 3A, the equivalent circuit model 252includes the primary power supply 253 connected to an inductor 254 and acapacitor 256 (representing the chip capacitance), and current sink 258(representing functional current consumption) of the IC exclusive ofvoltage islands. A switch 260 connects the primary power supply to avoltage island represented as a capacitor 262, and current sink 264. Asecondary or pre-charge power supply 270 is connected through aninductor 268 and a switch 266 for connecting the pre-charge power supply270 to the voltage island. The circuit 252 depicts the “00” state of thevoltage island where both the primary power supply 253 and thepre-charge power supply 270 are disconnected from the voltage island byswitches 260, 266, respectively.

Referring to FIG. 3B, the equivalent circuit model 274 depicts the “01”state of the voltage island where the pre-charge power supply 270 isconnected by switch 266 to the voltage island, and the primary powersupply 253 is not connected by switch 260. In this state, the largecapacitance of the voltage island becomes charged through voltage islanddedicated power supply 270 which may be shared across a plurality ofvoltage islands. Charging of capacitor 262 and leakage current 264,which increases with a rise in voltage across capacitor 262, does notaffect or create di/dt or associated noise on primary power supply 253and related chip capacitance 256.

Referring to FIG. 3C, the equivalent circuit model 276 depicts the “10”state of the voltage island where the pre-charge power supply 270 isdisconnected from the voltage island using switch 266, and the primarypower supply 253 is connected by switch 260. Voltage island capacitance262 was previously pre-charged to substantially primary supply voltage253 prior to connection of switch 260 and disconnect of switch 266,transient response on primary power supply bus 253 and associated chiplogic capacitance 256 is avoided.

Referring to FIG. 8, an exemplary control decoder 600 is shown whichsimplifies the generation and handling of the primary 204 and secondary220 header control inputs (shown in FIG. 3). The present invention maybe implemented with little or no additional control overhead compared tothe typical control requirements of an IC. As a minimum requirement, twoinputs with an associated decoder may be used to control the primaryheader, secondary header and logic boundary. It should be noted that thedecode circuitry of the present invention is intended to remainfunctional during power up/down of an associated voltage island,therefore, decoder logic is associated with the un-gated primary powersupply network of the IC.

Referring to FIG. 8, a global power supply domain 604 and voltage islandpower domain 640 are depicted. The voltage island includes primary powersupply connection 642 and secondary or pre-charge power supplyconnection 644. A decoder 610 communicates with the voltage island powerdomain 640. The decoder controls the different states of the voltageisland, boundary 614 a, and controls/generates further signals 616 a and616 b for other devices. Particularly, the decoder 610 receives thevoltage island state control signal inputs 630, 632 and 634 forprogramming the exemplary voltage island control states “01”, “10”, “11”and “10” via primary and secondary header control signal inputs 634 a,b,respectively, for at least one voltage island in the chip. The decodermay also control voltage island interface cells which controlpropagation of data and/or clock signals across boundary 614 a ofvoltage island power domain 600 via boundary control signal 636. Asfurther illustrated in FIG. 8, the decode function may be easilyexpanded to add control to other boundary functions such as separateclock and data gating at power-up/down, or power-down di/dt currentmitigation as shown in FIG. 11 using additional exemplary boundarycontrol lines 616 a and 616 b. The voltage island power domain 640includes the header 650, voltage island power rail 648 and connectionsfor receiving the primary power supply 642 and the secondary(pre-charge) power supply 644.

Referring to FIG. 9 an exemplary control flow logic table 700 accordingto the present invention is shown that depicts the programming state 730of the primary and secondary header control 724, 726, respectively, andvoltage island boundary 728 in response to decoder outputs including thevoltage island state control signal inputs to the decoder and includeisland control (0) 720, and island control (1) 722. The control flowbegins by preparing the decoder with island control (0) 720 and islandcontrol (1) 722 set to logic “00” state 702. In this state, the voltageisland is in full power-down mode 702 (e.g., after receipt of a powerdown request 758). Both the primary and secondary headers are disabled,as are the voltage island logic boundaries. In this state, valid outputsto logic outside the voltage island are maintained using voltage islandinterface cells, as are states within any state-keeping latches withinthe voltage island.

A Power up request is initiated 750 by programming the decoder with a“01” control logic state 704. The “01” control state 704 enables thesecondary header. With the secondary header enabled, the voltage islandis pre-charged to the voltage provided on the pre-charge power supplynetwork minus any IR drop caused by voltage island current draw. Thevoltage value of the secondary or pre-charge power supply issubstantially identical to the voltage value of the primary supplynetwork. As clocks and logic inputs to the voltage island remaindisabled in this mode, current is limited to any leakage within thevoltage island. As the secondary supply includes an inductive component,some ringing may occur on the secondary bus, however as the voltageisland remains in standby mode, sufficient time can be allowed prior tothe next state transition to allow settling. Additionally, because thecore is not connected to the primary supply network at this point, thestep response is not impressed on the primary power supply network, orthe functional circuits connected thereon. Timing between the “01” and“11” states may be gated by a counter or like delay function to allowsettling time, an analog comparison function to insure level, acombination thereof, or more complex logic functions as will bedescribed in greater detail in FIG. 10.

With state transition requirements satisfied, the system transitions 752to program the decoder with logic “11” state 706. At this time, thevoltage island is disconnected from the secondary supply (secondaryheader disabled) followed by enablement of the primary header. As thevoltage island Cs has been pre-charged to at or near the primary busvoltage, little or no di/dt results from primary header enablement,thus, ringing on the functional power supply network is avoided andfunctional circuits remain unaffected.

With supply switching complete, the state is moved 754 to program thedecoder with logic “10” 708 and the clock/data boundary is enabled. Thevoltage island enters functional mode at this point. In this example, asthe result of a power-down request, the control state transitions fromcontrol logic “10” state 708 to logic “00” state 702 disabling theprimary header and data/clock boundary, returning the voltage island topower down mode 702. One skilled in the art would recognize that theexample state table, control flow and decoder shown in FIG. 9 could beextended to separate clock and data enablement during power-up, separateclock, data and header disablement during power down, or add othercontrol ordering features.

Referring to FIG. 10, an exemplary higher-level control system for thevoltage island and control decode topology of the present invention isshown. A request facility 802 includes voltage power up/down requests810, which are forwarded to a central voltage island control facility850 which may manage one or multiple voltage islands. The controlfacility 850 manages state changes in accordance with the power up/downrequest, the state diagram to be implemented, and any monitor inputsreporting voltage island voltages or other parameters, and implementsthe state changes 860. In addition, where multiple voltage islands sharea single secondary or pre-charge power supply network, state transitionsof the various islands may be managed to limit loading on the secondarysupply in order to limit IR drop and insure that one island is notswitched from the secondary to primary supply network at a time whennoise may exist on the secondary network due to a recent charge-up eventon another island. This limits the potential delta between the primarysupply network and the voltage island pre-charge at the moment of statetransition. The control facility 850 may also manage voltage islandpower-up prioritization if multiple requests appear simultaneously usinga request stack 854. State reporting 820 and acknowledgement back to theisland power-up/down request facilities 802 is included within thecontrol facility 850.

It may also be desirable to reduce or eliminate any step response noiseassociated with voltage island power down. In most voltage islands,clocks are powered down and data boundaries stabilized prior to headerdisable. Shutdown of the clock network may produce enough change incurrent demand to initiate ringing in the power supply network. In thesesituations, the decoder and state diagram may be expanded to includeadditional power down sequencing.

Referring to FIG. 11, the circuit 900 having global/upper level powersupply 920, may be added to the voltage island header of the presentinvention to affect power down di/dt reduction at the time ofdisablement of the clock and/or data in the system. The circuit 900works in accordance with a state decoder to provide an initial leakagecurrent shunt at the time of a disable request, which is followed bysequential reduction in leakage current 910, 914 until full power down918 is reached. Thus, current demand associated with functional mode isstepped down slowly, even as the voltage island is disabled andstabilized. Sizing of the current sinks is made in accordance with theaverage functional current associated with island data/clock activityand a stepping plan. Bias for the switched current sources may come froma constant reference or a reference related to the primary power supplyvoltage (which relates to functional current). If a bias is notavailable, the current sink FETs may be connected in a diodeconfiguration.

As an alternative to the embodiment of FIG. 11, if the majority of ACcurrent draw comes from clock distribution and latches, the current stepdown function may be enabled using a system of segmented gating, inwhich each “power down control” line provides an enable/disable to asub-portion of the clock tree within the island. Thus, at power-down,successive power-down of each gated region is ordered or staged to stepdown current requirements of the island gradually. As a prelude togated/staged shutdown of the clock tree, any latched data to be saved tokeeper-latches is saved and the data boundaries of the island aredisabled to insure invalid data was not propagated outside the island.This same mechanism could be used to gradually increase the current drawof the island during power up, reducing the voltage island induced di/dtdue to clock enablement. In this instance, gated portions of the treewould sequentially be enabled prior to enabling the voltage island logicboundary and returning data from keeper latches to functional latches.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that changes in forms and details may be madewithout departing from the spirit and scope of the present application.It is therefore intended that the present invention not be limited tothe exact forms and details described and illustrated herein, but fallswithin the scope of the appended claims.

1. A system for reducing noise in an integrated circuit (IC) comprising:a primary circuit receiving a first power supply voltage for powering-upa voltage island circuit on the IC and functional circuitry on the IC; asecondary circuit receiving a second power supply voltage forpowering-up the voltage island; a programmed device for directing theprimary and secondary circuits for powering the voltage island circuitand functional circuitry, the programmed device directing the secondarycircuit to initially enable powering-up of the voltage island circuit toa pre-charged state, and subsequently disabling the secondary circuitwhile directing the primary circuit to enable powering of the functionalcircuitry during operation and the voltage island circuit, wherein astep response is avoided at the voltage island.
 2. The system accordingto claim 1 wherein the primary circuit and the secondary circuit includeprimary and secondary switches, respectively.
 3. The system according toclaim 2, wherein the primary switch includes a primary transistor andthe secondary switch includes a secondary transistor.
 4. The systemaccording to claim 1 wherein the primary circuit communicates with thefirst power supply via a first power path on the IC chip; and thesecondary circuit communicates with the second power supply via asecondary power path on the IC chip.
 5. The system according to claim 4wherein the second power supply connects to a plurality of secondarycircuits.
 6. The system according to claim 4 wherein the second powersupply connects to a plurality of secondary circuits on a plurality ofintegrated circuits.
 7. The system according to claim 1 furtherincluding a voltage island control circuit for managing circuit logic onat least one voltage island.
 8. A device for eliminating step responsepower supply perturbation during voltage island power-up/power-down onan integrated circuit, which comprises: an IC chip including a primarypower supply and a secondary power supply, and the IC chip includes atleast one voltage island; a primary header on the voltage island of theIC chip, the primary header communicating with the primary power supply;a secondary header on the voltage island of the IC chip, the secondaryheader communicating with the secondary power supply; and a controldecoder communicating with the IC chip and the voltage island forregulating the state of the primary and secondary headers.
 9. The deviceof claim 8 wherein the primary header on the voltage island of the ICchip communicates with the primary power supply via a primary headerpower path on the IC chip; and the secondary header on the voltageisland of the IC chip communicates with the secondary power supply via asecondary header power path on the IC chip.
 10. The device according toclaim 8 further including a header control system for managing theprimary and secondary header of the at least one voltage island.
 11. Thedevice according to claim 10 wherein the header control systemprioritizes initial power requests for multiple voltage islands.
 12. Thedevice according to claim 9 further including a header control systemfor managing a plurality of voltage islands which each include theprimary and secondary headers, and the plurality of voltage islandssharing the secondary power supply via the secondary header power pathsuch that the header control system limits loading of the secondarypower supply.
 13. The device of claim 8 wherein the control decodersequentially enables the secondary header on the voltage island whilethe primary header on the voltage island is disabled, and the controldecoder disables the secondary header on the voltage island and nearsimultaneously enables the primary header on the voltage island.
 14. Thedevice according to claim 8 wherein the secondary header power supplyconnects to a plurality of secondary headers.
 15. The device accordingto claim 8 wherein the secondary header power supply connects to aplurality of secondary headers on a plurality of IC chips.
 16. Thedevice according to claim 9 wherein the voltages on the primary headerand the secondary header are substantially the same.
 17. The deviceaccording to claim 13 wherein the control decoder prevents disabling thesecondary header on the voltage island and near simultaneously enablingthe primary header on the voltage island during a period when noise ison the secondary header or the secondary voltage is not substantiallyequal to the primary voltage.
 18. A method for eliminating step responsepower supply perturbation during voltage island power-up/power-down onan integrated circuit, comprising: providing an IC chip communicatingwith a primary power supply, and the IC chip including at least onevoltage island; providing a primary header on the voltage island of thechip, the primary header communicating with the primary power supply viaa primary header power path; providing a secondary header on the voltageisland of the chip, the secondary header communicating with a secondarypower supply via a secondary header power path; providing a controldecoder communicating with the IC chip and the at least one voltageisland; requesting an initial power-up of the voltage island; enablingthe secondary header using the control decoder; disabling the secondaryheader using the control decoder; enabling the primary header using thecontrol decoder;
 19. The method of claim 18 wherein the secondary headeris disabled and the primary header is enabled near simultaneously. 20.The method of claim 18 further comprising a power down sequenceincluding a series of stepped reductions in current demand from thevoltage island when the primary header is switched from enabled to adisabled mode using the control decoder.